In the papermaking industry, disk refiners are utilized to refine stock as an initial step in the papermaking process. Stock flows into an inlet of the refiner and then passes between a pair of refiner disks, one of which rotates with respect to the other disk, to refine the stock.
Initially, rather massive fibrous clumps, typically in the form of wood chips, are disposed in a liquid stock slurry such that the consistency of the stock is thick with fibrous matter and referred to as high consistency stock. To help soften the chips so they more easily break apart during refining, they are heated and chemically treated in a tank called a digester before refining.
High consistency stock is refined by refiners specifically setup to handle breaking up such large chips apart into smaller components. Refiners are typically staged so as to progressively break the fibrous matter into increasingly smaller components with the desire that the stock will be almost entirely composed of individual fibers entrained in liquid by the time the stock reaches a paper machine or fiber product making apparatus. Liquid is typically added to the stock at each stage to dilute the fibrous matter so it can more easily pass through increasingly narrower refiner disk gaps required to refine the fibrous matter into ever-smaller components.
As the fibrous matter becomes more diluted and smaller in size, the consistency of the stock is correspondingly reduced. At some point, the percentage of fibrous matter becomes six percent or less, and the stock is defined as being low consistency stock. One desired goal of refining that takes place at or after this point is to refiner the fibrous matter into individual fibers that are fibrillated so they more tightly engage each other when the fibers are formed into a sheet of paper or some other like fiber product. This increases finished product strength, while enabling ever-higher production rates to be achieved.
Stock feed assist devices have been employed in the past in high consistency refining applications to help force the relatively thick stock into the gap between refiner disks of a high consistency refiner. Since fibrous matter of high consistency stock consists of relatively large fibrous components, typically wood chips, refining of high consistency stock usually generates so much heat that a considerable amount of steam is produced. Such feed assist devices are also employed to help overcome the opposition to stock flow due to the pressure of steam seeking to escape the refining zone against the direction of flow. Some examples of feed assist devices used in high consistency refiners are disclosed in U.S. Pat. Nos. 5,076,892, 5,383,608, and 5,626,300.
It is believed that feed assist has not been heretofore been used in low consistency refining applications. Since low consistency stock is comprised almost entirely of liquid and a small amount of fiber, steam does not adversely impact the flow of entering stock anywhere near the same degree as it does in high consistency refining, employing any kind of feed assist in a low consistency refiner application was not heretofore believed to significantly impact low consistency refining.
One type of refiner that is used in low consistency refining applications is a double disk refiner. A double disk refiner has an inlet through which stock flows into a first refining zone that is located closest to the inlet and a second refining zone located downstream of the first refining zone. A double disk refiner includes a rotor that carries a pair of refining surfaces that face away from each other with each of these refining surfaces, in turn, opposing a stationary refining surface, defining refining zones therebetween. The rotor includes a perforate hub through which some stock entering the refiner must flow to reach the second refining zone, which is located downstream of the hub.
As a result of this construction, low consistency stock flow conditions are complex and believed not heretofore fully understood. For example, stock passing through the perforate hub drops in pressure. This is believed to occur at least in part because some of the stock flowing toward to second refining zone impacts the hub before it reaches the second refining zone. This dissipates some of the energy of the stock, which thereby decreases its velocity before it enters the second refining zone. As such, its velocity is less than the velocity of the stock flowing into the first refining zone. Additionally, the fluid shearing action of the hub rotating generally perpendicular to stock flow, creates flow disturbances that include wakes, flow-opposing cavitation, turbulence, as well as localized pressure differences in the stock along the hub that can further reduce the rate of stock flow into the second refining zone.
It is also believed not heretofore understood the full extent how such flow conditions and the double disk refiner geometry also impacts the distribution of fiber of low consistency stock entering the refiner. For example, despite the fact that no more than six percent of low consistency stock is comprised of fiber, it has not been heretofore well understood about how to best disperse fiber that tends to agglomerate in double disk refiners between the stock inlet and both refining zones as a result.
Thus, in the past, performance of double disk refiners in low consistency refining applications has been less than optimal. For example, the aforementioned fiber agglomeration causes fiber entering each refining zone to be nonuniformly distributed, which, for example, typically manifests itself in an undesirably high amount of shives. Shives are bundles of fibers still bound together (such as by lignin), which are discharged by the refiner. These are undesirable as they are much larger than desired and tend not to be fibrillated enough to adequately engage other surrounding fibers when sheet forming takes place.
In the past, a double disk refiner of Sprout-Bauer, Inc., marketed under the trade name Twin-Flo III, was equipped with a pair of agitator assemblies carried on the rotor drive shaft that were each intended to break up clumps in low consistency stock. Each agitator assembly is a circular collar clamped on the shaft for rotation in unison therewith having a pair of square tabs that each extends out from the collar into stock located adjacent one of the refining zones of the double disk refiner. One agitator assembly is located at the end of a stock inlet conduit and just upstream of both refining zones. The second agitator assembly is located downstream of both refining zones in a stock-receiving pocket.
Unfortunately, rotation of the square tabs of each agitator assembly creates retarding eddies and turbulence that can adversely impact stock flow, which can actually cause clumping. In particular, the agitator assembly located upstream of both refining zones actually decreases stock flow and can actually cause stock backflow out of the first refining zone back toward the inlet. The shape of each of agitator assembly tab and the orientation each tab relative to the intended direction of stock flow impedes flow to both refining zones and also has virtually no impact in preventing the hub from impeding flow to the second refining zone. As a result, the volume of shives outputted by a low consistency double disk refiner so equipped remains undesirably high, energy efficiency is less than optimal as a result of the increased energy dissipated by each agitator assembly, and refiner throughput via both refining zones is less than ideal.
What is needed is an improved double disk refiner, low consistency stock arrangement for such a refiner that helps maximize uniformity of the distribution of fiber in stock entering each refining zone of the refiner, and an improved low consistency stock refining method.